Saw-tooth current oscillation generator



March 6, 195] A. J. BENNETT ETAL SAWTOOTH CURRENT OSCILLATION GENERATOR Filed Jan. 27.- 1948 I Srnwaws D.C VOLTAGE cm 9 Inventor Arihur 861m eH' a-nd Ladislav Lax Attorneys Patented Mar. 6, 1951 SAW -TGOTH CURRENT OSCILLATION GENERATOR Arthur J. Bennett and Ladislav Lax, Cambridge, England, assignors to Pye Limited, Cambridge,

England, a British company Application January 27, 1948, Serial No. 4,588 In Great Britain February 1,1947

3 Claims. (Cl. 250-36) The present invention relates to a sawtooth current oscillation generator employing a single valve which serves the dual purpose of generating of sawtooth current oscillations and as an amplifier thereof.

This invention is directed to the linearisation of the oscillations in a single valve sawtooth current oscillation generator in which a substantially sawtooth current is produced in a network composed mainly of inductances.

The invention will be more clearly understood with reference to the accompanyin drawings, in which Figure 1 shows a basic circuit diagram of a sawtooth current generator;

Figure 2 shows a graph and Figure 3 shows a circuit arrangement according to the invention.

In order to produce a sawtooth current in a network composed only of inductances it is necessary to apply a periodically interrupted constant voltage to the terminals thereof. Figure 1 shows a circuit arrangement for this purpose, which is suitable for producing the line oscillations in television receiving apparatus.

The circuit comprises a multi-grid valve V having two electrodes gl, g2 arranged between the cathode and anode, an inductance L being connected in the anode lead in which the amplified sawtooth oscillations are produced. The inductance L may comprise the primary winding of the line scan transformer S, the secondary of which is connected to the line deflecting coils D of the cathode ray tube.

The transformer T provides a positive feedback between the two grid electrodes gl, 92. If the feedback is tight the dynamic characteristics of the grid volts (Eg2) to grid current (192) of the grid g2 will be as shown in Figure 2. This shape of characteristic, especially as regards the sections ab and bc, may be easily produced by employing a suitable transformer ratio and tightness of coupling. Between th points a and b the impedance of the electrode g2 is positive and thus Igz increases with EgZ. At point b the impedance increases. quickly to infinity and 102 remains substantially constant with increase of E92. The duration of the portion 120 is determined by the inductances and resistances in the circuit and the biases on the diiierent electrodes.

At point the impedance of electrode 92 becomes negative, and that section of the circuit begins to oscillate. The voltage of electrode g2 increases quickly and that of electrode gl decreases until it finally cuts off the flow of elec- 2 trons in the valve. The frequency of these oscillations is determined by the capacity Cl across the primary and secondary of the transformer T produced by the inter-electrode capacities'of the valve and the self-capacities of the transformer T. The frequency may be controlled by variations of the resistance R I.

During the period of the d'abcd portion of the complete cycle, the potential on electrode gl is more positive than the cathode or established bias voltage. The resistance R2 and condenser C2 serve to adjust the voltage on the electrode 92 to the correct value, as the impedance on g2 is positive, and the voltage on g! is positive, and the valve is conducting.

By correct adjustment the section ab may be made very steep so that IgZ suddenly rises to a high value and remains constant. As the voltage on electrode 92 increases, that on electrode gl decreases and changes the impedance of the anode circuit.

The above mentioned action of the theoretical circuit does not take into account the resistance elements in the inductances or the internal impedance of the valve.

In order to prevent an increase of the anode impedance due to the decrease in voltage of electrode gl, a third winding is, as shown in Figure 3, introduced on the feed back transformer T and connected in the cathode circuit in such a way that the potential changes on the electrode gl are followed by that on the cathode. This further increases the potential difference between the cathode and electrode 92, and partly counteracts the effects of finite resistances in the anode circuit. In order to prevent unwanted ringin in the anode circuit, a diode may be connected in various ways in that particular part of the circuit. In the circuit shown in Figure 3 the resistance R3 may be used as a frequency control.

The frequency of the oscillations generated by either embodiment may be synchronised by the application of negative pulses to the electrode 9! or positive pulses to the electrode g2.

We claim:

1. A sawtooth current oscillation generator comprising a valve having at least two grid electrodes between the cathode and the anode, a source of high tension supply, a transformer having three mutually coupled windings, that one of the said two grids nearer to the cathode being connected to the negative high tension supply through one of said windings, the other grid of the valve being connected to the high tension positive supply through a second winding on said 3. A generator according to claim 1, having a transformer and a series resistance, a condenser resistance included in the connection from the connecting the point of junction of said resistgrid electrode nearer to the cathode to ground. ance and said second Winding to a point of fixed potential, said first and second windings being 5 ARTHUR J. BENNETT. coupled to provide positive feed back between LADISLAV LAX.

said grids, the cathode of the valve being connected through the third winding on the trans- REFERENCES CITED former to the negative high tension Supply, Said The following references are of record in the third winding being coupled to the other wind- 10 fil f this patent;

ings to apply potentials to the cathode in the same sense as the potentials on the grid elec- UNITED STATES PATENTS trode nearer to the cathode and an output circuit Number Na e Date connected between the anode and the high tenv2, Fa l et a1 Aug. 20, 9 0

sion positive supply. 15 2,303,924 Faudell Dec. 1, 1942 2 A generator according to claim 1, having a FOREIGN PATENTS variable resistance included in the connection between the cathode and ground to provide a Number Country Date variable frequency control. 493,142 Great Britain Oct. 5, 1938 

